1. Field of the Invention
The present invention relates to a rechargeable battery. In particular, the present invention relates to a rechargeable battery that has an improved mounting structure for an electrode terminal and a second protective device used therein that ensures the stability of the rechargeable battery.
2. Description of the Prior Art
Rechargeable batteries are being developed to power lightweight portable wireless electronic devices such as video cameras, mobile telephones, and portable computers. Commonly used rechargeable batteries include, for example, nickel cadmium batteries, nickel hydrogen batteries, nickel zinc batteries, and lithium rechargeable batteries. Lithium rechargeable batteries are widely used in cutting-edge electronics applications because they are rechargeable, they can be made in a compact size with large capacity, they have a high operation voltage, and they have a high energy density per unit weight. These properties enable lithium batteries to meet the demands of complex electronic devices that incorporate numerous functions.
A typical rechargeable battery includes an electrode assembly, which includes positive and negative electrode plates, and a separator. The electrode assembly is housed in a metallic can. An electrolyte is injected into the can, which is sealed to form a bare cell. The bare cell generally has an electrode terminal that is positioned on top of it and is insulated from the can. The electrode terminal acts as one electrode of the battery and the battery can itself as the other electrode.
After the bare cell is sealed, a safety device such as a second protective device including a positive temperature coefficient (PTC) thermistor and a protective circuit module (PCM) is connected to the top of the bare cell and is encased in a battery pack or molded with a resin to form a rechargeable battery. The safety apparatus is connected to positive and negative electrodes and interrupts electrical current when the temperature or voltage of the battery rises abruptly due to overcharge or over-discharge in order to avoid damage to the battery.
FIG. 1 is a partial sectional view showing a bare cell of a rechargeable battery according to the prior art.
The bare cell 10 of the rechargeable battery includes a can 20, an electrode assembly 22, and a cap assembly 30. The bare cell 10 may also include a second protective device 40 on the top thereof, depending on the construction of the rechargeable battery.
Referring to FIG. 1, the can 20 is a metallic container with a cuboid shape with its top open. The can 20 is preferably made of aluminum or an aluminum alloy, which is a light metal with good conductivity and excellent corrosion resistance. The can 20 comprises the electrode assembly 22, which includes a positive electrode plate 23, a separator 24, a negative electrode plate 25, and an electrolyte. After the electrode assembly 22 is inserted into the can 20 through its top opening, the top opening is sealed by the cap assembly 30.
The cap assembly 30 is provided with a planar plate-type cap plate 31 with a size and shape corresponding to those of the top opening of the can 20.
The electrode assembly 22 is formed by winding positive and negative electrode plates 23 and 25, respectively, with a separator 24 interposed between them. The positive electrode plate 23 is coupled to the cap plate 31 via a positive electrode tab 26 and the negative electrode plate 25 is coupled to a negative terminal 32 of the cap plate 31 via a negative electrode tab 27. The can 20 is electrically insulated from the negative terminal 32 and acts as a positive terminal. After the cap assembly 30 is welded to the top of the can 20, an electrolyte is injected through an electrolyte injection hole 36 of the cap plate 31. The electrolyte injection hole 36 is sealed by a cap 37 formed by press-fitting a ball.
The cap plate 31 is preferably made of the same material as the can 20, i.e., aluminum or an aluminum alloy to provide improved welding to the can 20. The cap plate 31 has a terminal through-hole formed in its center through which the negative terminal 32 can pass. A tube-shaped gasket 33 is positioned on the exterior of the negative terminal 32, which extends through the center of the cap plate 31 for electrical insulation between the negative terminal 32 and the cap plate 31. An insulation plate 34 is positioned beneath the cap plate 31 near the terminal through-hole of the cap plate 31. A terminal plate 35 is positioned beneath the insulation plate 34.
The second protective device 40 is located on the top surface of the cap plate 31 and is positioned between the negative terminal 32 and a protective circuit module (not shown in the drawing). A PTC thermistor or a thermal fuse may be used as the rechargeable protective device 40.
Conventional rechargeable batteries that are configured as discussed above have a design flaw in that the second protective device is mounted on top of the cap plate, away from the interior of the can. This results in reduced temperature sensitivity of the protective device.
In addition, the head portion of the electrode terminal protrudes from the top of the cap plate and is not flush with the second protective device. This makes it difficult to connect the second protective device to the electrode terminal. The electrode terminal may also rotate while welding or otherwise attaching the second protective device to the electrode terminal. This may weaken or even break the welded portion between the electrode terminal and the negative electrode tab.